The present invention relates to an input/output device for a programmable controller, and more particulary to an input/output device capable of exchanging the input/output data with the main unit of the controller by means of serial data transmission.
In recent years, relatively large-scale centralized control systems using programmable controllers are frequently employed in control systems for ordinary machine plants. In such a centralized control system, various kinds of input devices such as limit switches, temperature switches, proximity switches, photoelectric switches and various kinds of output devices such as motors, plungers, electromagnetic valves and so forth are respectively connected to the central controller with individual signal conductors in many cases. Where this kind of system has many input/output devices that are dispersedly located in a relatively large space, the wiring space for the connections between the input/output devices and the central controller and the wiring cost have become great problems and, thus, currently it is strongly advantageous to simplify signal transmission by properly applying multiplex transmission.
Various kinds of multiplex transmission systems are conventionally known, which are utilized for the transmission of the input/output data in the programmable controller. However, in the conventional multiplex transmission system, an intrinsic address is assigned to each terminal of the system and circuit for judging the address which is provided at each terminal, but its transmission control procedure including an address-discriminating circuit is very complicated and sophisticated, thereby making this system very expensive. Of course, it can be advantageous to employ such sophisticated circuitry, but the conventional system has many functions not required for transmission of the input/output data in the programmable controller and, thus, as a result is not able to properly respond to the request stated above.